A preliminary search of the Tsarnaev brothers' cell phones and computers finds no indication that the two suspected Boston Marathon bombers had any accomplice in the bombing, a counterterrorism source briefed on the FBI's investigation told NBC News.

The investigation continues, but FBI officials right now are increasingly confident that "nobody else was involved," according to the source.

NJ Sister of Boston Bombing Suspects Stays in Home

An attorney for the New Jersey sister of the alleged Boston bombers told reporters Ailina Tsarnaeva has been holed up with her husband and baby in their West New York apartment building, and it was "very difficult" for her. Brian Thompson reports. (Published Tuesday, Apr 23, 2013)

The magazine has twice included articles on how to build pressure cooker bombs, the same type that was used in the twin Boston Marathon attack that killed three people and wounded hundreds more, according to NBC News.

The 19-year-old's older brother Tamerlan Tsarnaev, 26, was an ardent reader of jihadist websites and extremist propaganda, U.S. officials said Tuesday, adding another piece to the body of evidence they say suggests the pair were motivated by an anti-American, radical version of Islam.

As he lay in his hospital bed with a gunshot wound to the throat, Dzhokhar Tsarnaev was charged on Monday with carrying out the bombing with Tamerlan, who died last week in a gunbattle. Tsarnaev could get the death penalty.

Interrogators questioned him at the hospital, letting him write down his replies, and his answers led them to believe he and his brother were motivated by religious extremism but appeared to have no major terrorist group connections, said U.S. officials, who spoke on condition of anonymity because they were not authorized to discuss the investigation publicly.

Boston University Mourns Grad Student Killed in Bombings

However, the written communication precluded back-and-forth exchanges often crucial to establishing key facts, officials said. They warned that they were still trying to verify what Tsarnaev told them and were poring over his telephone and online communications.

On Tuesday, two officials told The Associated Press the older brother frequently looked at extremist sites, including Inspire which is produced by al-Qaida's Yemen affiliate and has endorsed lone-wolf terror attacks.

Also Tuesday, family, friends and colleagues gathered to pay their final respects to Sean Collier, the Massachusetts Institute of Technology police officer who authorities say was ambushed and killed by the Boston Marathon bombing suspects. And a private funeral was held for 8-year-old Martin Richard, the youngest of the three people killed in the bombing. In a statement, the boy's family called it "the most difficult week of our lives."

Dzhokhar Tsarnaev, whose condition was upgraded Tuesday from serious to fair, was charged with using and conspiring to use a weapon of mass destruction. The University of Massachusetts-Dartmouth student was accused of joining with his brother in setting off the shrapnel-packed pressure-cooker bombs that killed three people and wounded more than 260 on April 15.

The brothers are ethnic Chechens from Russia who had been living in the U.S. for about a decade.

The next step in the legal process against Tsarnaev is likely to be an indictment, to which federal prosecutors could add new charges. State prosecutors have said they expect to charge Tsarnaev separately in the killing of the MIT officer.

Federal public defender Miriam Conrad, whose office has been asked to represent Tsarnaev, asked that two death penalty lawyers be appointed to represent Tsarnaev, "given the magnitude of this case."

A probable cause hearing — at which prosecutors will spell out the basics of their case — was set for May 30. According to a clerk's notes of Monday's proceedings in the hospital, U.S. Magistrate Judge Marianne Bowler indicated she was satisfied that Tsarnaev was "alert and able to respond to the charges."

Tsarnaev did not speak during the proceeding, except to answer "no" when he was asked if he could afford his own lawyer. He nodded when asked if he was able to answer some questions and whether he understood his rights.

Conrad declined to comment when contacted by The Associated Press.

The criminal complaint outlining the allegations shed no light on the motive for the attack.

In the criminal complaint, investigators said Dzhokhar Tsarnaev and his brother each placed a knapsack containing a bomb in the crowd near the finish line of the 26.2-mile race. The FBI said surveillance-camera footage showed Dzhokhar manipulating his cellphone and lifting it to his ear just moments before the two blasts.

After the first blast, a block away from Dzhokhar, "virtually every head turns to the east ... and stares in that direction in apparent bewilderment and alarm," the complaint says. But Dzhokhar Tsarnaev, unlike practically everyone else around him, appeared calm, the FBI said.

He then quickly walked away, leaving a knapsack on the ground; about 10 seconds later, a bomb blew up at the spot where he had been standing, the FBI said.

The FBI did not say whether he was using his cellphone to detonate one or both of the bombs or whether he was talking to someone.

Among the details in the affidavit:

— Dzhokhar Tsarnaev had gunshot wounds to the head, neck, legs and hands when he was captured hiding out in a boat in a backyard in the Boston suburb of Watertown, authorities said.

— One of the brothers — it wasn't clear which one — told a carjacking victim during their getaway attempt: "Did you hear about the Boston explosion? I did that."

— The FBI said it searched Tsarnaev's dorm room at the University of Massachusetts-Dartmouth on Sunday and found BBs as well as a white hat and dark jacket that look like those worn by one of the suspected bombers in the surveillance photos the FBI released a few days after the attack.